1. Field of the Invention
The present invention in part relates to the discovery that the T1R receptors assemble to form functional taste receptors. Particularly, it has been discovered that co-expression of T1R1 and T1R3 results in a taste receptor that responds to umami taste stimuli, including monosodium glutamate. Also, it has been discovered that co-expression of the T1R2 and T1R3 receptors results in a taste receptor that responds to sweet taste stimuli including naturally occurring and artificial sweeteners.
Also the present invention relates to the use of hetero-oligomeric taste receptors comprising T1R1/T1R3 and T1R2/T1R3 in assays to identify compounds that respectively respond to umami taste stimuli and sweet taste stimuli.
Further, the invention relates to the construction of cell lines that stably or transiently co-express a combination of T1R1 and T1R3; or T1R2 and T1R3; under constitutive or inducible conditions.
The use of these cell lines in cell-based assays to identify umami and sweet taste modulatory compounds is also provided, particularly high throughput screening assays that detect receptor activity by the use of fluorometric imaging.
2. Description of the Related Art
The taste system provides sensory information about the chemical composition of the external world. Mammals are believed to have at least five basic taste modalities: sweet, bitter, sour, salty, and umami. See, e.g., Kawamura et al., Introduction to Umami: A Basic Taste (1987); Kinnamon et al., Ann. Rev. Physiol., 54:715-31 (1992); Lindemann, Physiol. Rev., 76:718-66 (1996); Stewart et al., Am. J. Physiol., 272:1-26(1997). Each taste modality is thought to be mediated by a distinct protein receptor or receptors that are expressed in taste receptor cells found on the surface of the tongue (Lindemann, Physol. Rev. 76:718-716 (1996)). The taste receptors that recognize bitter, sweet, and umami taste stimuli belong to the G-protein-coupled receptor (GPCR) superfamily (Hoon et al., Cell 96:451 (1999); Adler et al., Cell 100:693 (2000)). (Other taste modalities are believed to be mediated by ion channels.)
G protein-coupled receptors mediate many other physiological functions, such as endocrine function, exocrine function, heart rate, lipolysis, and carbohydrate metabolism. The biochemical analysis and molecular cloning of a number of such receptors has revealed many basic principles regarding the function of these receptors. For example, U.S. Pat. No. 5,691,188 describes how upon a ligand binding to a GPCR, the receptor undergoes a conformational change leading to activation of a heterotrimeric G protein by promoting the displacement of bound GDP by GTP on the surface of the Gα subunit and subsequent dissociation of the Gα subunit from the Gβ and Gγ subunits. The free Gα subunits and Gβγ complexes activate downstream elements of a variety of signal transduction pathways.
This invention relates to the three-member T1R class of taste-specific GPCRs. Previously, the T1R receptors were hypothesized to function as sweet taste receptors (Hoon et al., Cell 96:541-51 (1999); Kitagawa et al., Biochem Biophys Res. Commun. 283:23642 (2001); Max et al., Nat. Genet. 28:58-63 (2001); Montmayeur et al., Nat. Neurosci. 4: 412-8 (2001); Sainz et al., J. Neurochem. 77: 896-903 (2001)), and Nelson et al. (2001) have recently demonstrated that rat T1R2 and T1R3 act in combination to recognize sweet taste stimuli. The present invention relates to two discoveries. First, as is the case for rat T1R2/T1R3, human T1R2 and T1R3 act in combination to recognize sweet taste stimuli. Second, human T1R1 and T1R3 act in combination to recognize umami taste stimuli. Therefore, T1R2/T1R3 is likely to function as a sweet taste receptor and T1R1/T1R3 is likely to function as an umami taste receptor in mammals. The likely explanation for the functional co-dependence of T1R1 and T1R3 and the function co-dependence of T1R2 and T1R3 is that, like the structurally related GABAB receptor (Jones et al., Nature 396: 5316-22 (1998); Kaupmann et al., Nature 396: 683-7 (1998); White et al., Nature 396:679-82 (1998); Kuner et al., Science 283: 74-77 (1999)), T1Rs function as heterodimeric complexes. However, it is alternatively possible that this functional co-dependence reflects a necessary but transient interaction that ultimately produces functionally independent monomeric or homomultimeric taste receptors.
The identification of characterization of taste receptors which function as sweet and umami receptors is significant as it will facilitate the use of these receptors in assays for identifying compounds that modulate (enhance or block) sweet and umami taste. These compounds would be useful for improving the taste and palatability of foods, beverages, medicinals for human or animal consumption. Particularly, an assay that utilizes a functional sweet receptor would allow the identification of novel sweeteners.